Turbine wheels are typically cast in rough form and then machined into a final shape. It is known to statically measure the balance of a cast, unfinished radial turbocharger turbine wheel by affixing it in a balance-test jig and statically finding a center of mass in two radial dimensions (two lateral dimensions normal to one another and normal to an anticipated axis of rotation). It is further known as mass centering to drill a small feature (a center drill) such as a conical hole on an axial end of the wheel at the two-dimensional center of mass to establish a center of rotation for the wheel. Machining of the wheel can then be conducted with respect to that center of rotation.
It is also known to use a balance-test jig to radially clamp around the blades of the turbine wheel, and also onto a backplate of a turbine wheel at a datum plane. Using this in a rotating fixture, centering measurements may be made at one axial plane of the wheel. Mass centering can then be done. While more accurate than a static measurement, this does not lead to accuracy and repeatability, as unfinished wheels will not typically have the geometric center be the mass center. Therefore, each time the wheel is mounted in the balance-test jig, it may have a different imbalance.
Extensive machining of small turbine wheels is difficult and expensive. The more machining that is required, the greater the level of difficulty and the greater the cost. When outsourcing the manufacture of cast, unfinished turbine wheels, it may therefore be desirable to set limits on the imbalance of the purchased wheels, and to enforce those limits with product testing. Great accuracy is desirable when testing such imbalance limits, and this level of accuracy is particularly difficult with very small turbine wheels.
One difficulty in such testing is that balance-test jigs for holding a turbine wheel are typically characterized by their own levels of imbalance. This is imbalance is exacerbated by the locking mechanisms of the balance-test jigs, which may produce a different imbalance every time it is locked on to a wheel.
Accordingly, there has existed a need for a turbocharger wheel balance-test jig that can very accurately test the imbalance of very small radial turbocharger wheels. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.